In known types of nuclear reactors, such as boiling water reactors (BWR), the reactor core comprises a plurality of fuel assemblies arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel and submerged in water, which serves as both a coolant and a neutron moderator. A plurality of control rods containing neutron absorbing material are insertable in gaps between the fuel assemblies to control the reactivity of the core. Each fuel assembly includes a flow channel through which water is pumped upwardly from a lower plenum to an upper plenum. To monitor the power density of the core, it is common practice to distribute neutron detectors both radially and axially throughout the core. The signals from these neutron detectors are utilized to monitor core conditions and to initiate corrective actions, including reactor shutdown (SCRAM), in the event of a detected abnormality.
One reactor abnormality that has come under close scrutiny due to recent events is thermal-hydraulic instability. As water is pumped upwardly through the fuel assembly flow channels, vaporization occurs. The resulting vapor bubbles are in constant motion, ever expanding and contracting. This produces variations in the two-phase fluid flow through the channels. If these flow variations are not dampened or suppressed by normal flow losses due to friction, they can build into sustained oscillations. Since the fluid is also a neutron moderator, flow oscillations will result in neutron flux oscillations and thus power oscillations along the vertical length of the fuel assemblies. With recent changes in plant operating modes and fuel neutronic and heat transfer characteristics, such thermal-hydraulic induced power oscillations could conceivably exceed minimum critical power ratio (MCPR) safety limits.
Such thermal-hydraulic instabilities, which have the potential of exceeding stability margins only under high power and low coolant flow operating conditions, can occur symmetrically throughout the core (core-wide oscillations) or asymmetrically, wherein core flow and consequent neutron flux in various regions of the core oscillate in out of phase relation (regional oscillations).
Existing in-core power monitoring instrumentation has been largely directed to monitoring average power by averaging the signals from selected neutron detectors widely distributed within the core. While such average power range monitoring (APRM) systems can detect and initiate action to suppress unacceptably high core-wide neutron flux oscillations, they do not reliably detect regional oscillations, since averaging detector signals that are relatively out of phase results in substantial cancellation.
In commonly assigned, copending Watford et. al. application entitled "Oscillation Power Range Monitoring System and Method for Nuclear Reactors", Serial No. 07/644,349, filed Jan. 22, 1991, a system and method are disclosed for reliably detecting both core-wide and regional neutron flux oscillations due to thermal-hydraulic instabilities. Upon detection of the onset of such oscillations, a signal is generated to initiate an appropriate reactor control function to suppress the oscillations before they can build to magnitudes exceeding an established reactor stability margin.
To this end, detector signals from local power range monitoring (LPRM) strings, radially distributed throughout the reactor core, are selectively assigned to a core-wide array of oscillation power range monitoring (OPRM) cells to develop unique cell output signals representative of the average neutron flux density or power existing at highly localized regions distributed throughout the reactor core.
Four oscillation power range monitoring (OPRM) channels are respectively assigned to selected groups of OPRM cells distributed throughout the core in geographically overlapping and partially lapping relations. The output signal of each OPRM cell is repetitively sampled and processed to detect oscillations thereof which are characteristic of the onset of a thermal-hydraulic instability. When an oscillation of an amplitude meeting certain setpoint and frequency criteria is detected, the assigned OPRM channel is tripped. If at least two OPRM channels are tripped, an unacceptable thermal-hydraulic instability is reliably indicated, and an automatic suppression function (ASF) is initiated to suppress the oscillations.
As noted above, thermal-hydraulic instabilities resulting in neutron flux oscillations of concern (particularly asymmetrical oscillations) have been found to occur only when a BWR reactor is operating under high power/low coolant flow conditions. Thus, it has been proposed to restrict reactor operation to the stable region of the reactor's power/flow map, wherein the power/flow ratio is sufficiently low to preclude thermal-hydraulic instabilities. If entry into the potentially unstable region inadvertently occurs, measures are automatically instituted to exit the region, e.g., either reduce power or increase coolant flow. Although entry into the unstable power/flow region is never planned, certain plant operating modes may cause a BWR reactor to operate close to the boundary line defining the two regions, and, under certain conditions, excursions into the potentially unstable region can occur.